1. Field of the Invention
The present invention relates to a wireless transmitter and amplifier to be used in a wireless communication system.
2. Description of the Background
In recent years, various techniques have been proposed to improve the data transmission rate in wireless communication systems. One of them is a technique called MIMO (Multi-Input Multi-Output).
The MIMO is a technique of transmitting a radio wave by using a plurality of analog transmitting series each including an antenna and receiving a radio wave by using a plurality of receiving series in the same way. The transmission rate can be increased using a phenomenon called multipath fading that a radio wave is subject to diffused reflection by a building or the like when the radio wave propagates through the space.
As an example, a conventional configuration of a MIMO transmitter having two transmitting series includes a baseband unit, first and second modulation units which modulate a baseband modulation signal output from the baseband unit, first and second power amplifiers which amplifies modulated signals respectively output from the first and second modulation units, and first and second antennas which radiate signals output from the first and second power amplifiers as radio waves.
The MIMO is the technique of increasing the transmission rate by using the phenomenon called multipath fading as described above. If the transmitter and the receiver are present in open places where the visibility is good, therefore, the transmission rate does not increase in some cases. In such a case, it might be effective to use only one transmitting series without utilizing the MIMO, from the viewpoint of the power dissipation as well because the operating circuit is reduced. Furthermore, in an area where a conventional system is being provided, it might become necessary to notify terminals that are used in the conventional system that the own machine will operate in MIMO hereafter. In such a case, even a transmitter for MIMO having a plurality of transmitting series is required to also operate by using only one transmitting series and communicate. In the ensuing description, the time when the MIMO transmitter operate in MIMO by using a plurality of transmitting series is referred to as “at the time of MIMO transmitting,” and the time when the MIMO transmitter transmits a signal by activating only one transmitting series is referred to as “at the time of non-MIMO transmitting.”
For making the signal level received by a receiver even at the time of non-MIMO transmitting equal to that at the time of MIMO transmitting, it is necessary to output signal from the transmitter at the time of MIMO transmitting equal to that at the time of non-MIMO transmitting. In such a case, a power amplifier to be used at the time of non-MIMO transmitting among power amplifiers included in a plurality of transmitting series that form a MIMO transmitter is required to have a capability of outputting a signal having a power level to be output by the transmitter in a specification range of distortion prescribed by the wireless communication system.
It is now supposed that the maximum output power level to be output by the transmitter is X [W] and N transmitting series are used at the time of MIMO transmitting. A power amplifier to be used at the time of non-MIMO transmitting must be able to output a signal with power of X [W] while satisfying distortion specifications prescribed by the wireless system. On the other hand, a maximum output power level to be output by each power amplifier at the time of MIMO transmitting becomes X/N [W] if the same power amplifiers are utilized. At the time of MIMO transmitting, it suffices that each power amplifier outputs a signal with maximum power of X/N [W] while satisfying distortion specifications prescribed by the wireless system. A difference in the maximum output level of one power amplifier between the non-MIMO transmitting and the non-MIMO transmitting becomes (N−1)×X/N [W].
In other words, if a MIMO transmitter having N transmitting series operates in non-MIMO, a power amplifier is required to output a signal having a signal level that is N times as high as that at the time of MIMO transmission.
In general, as the output level approaches a saturation output level in power amplifiers, the operation efficiency rises, but the distortion characteristics are degraded. On the other hand, as the output level becomes lower than the saturation level, the distortion characteristics are improved, but the operation efficiency is degraded. Causing a power amplifier to operate with the output of the power amplifier lowered below its saturation output level in order to satisfy distortion characteristics is referred to as “back-off.” The difference between the saturation output level and the actual output level is referred to as “back-off level.” If the power amplifier is brought into backoff operation, the operation efficiency is degraded. In a MIMO transmitter which operates in non-MIMO as described above, therefore, the power amplifier operate inefficiently when executing MIMO transmission.
In order to make the relation between the backoff level of the power amplifier and the operation efficiency comprehensible, the above-described conventional MIMO transmitter (a baseband unit, first and second modulation units, first and second power amplifiers, and first and second antennas) is taken as an example. The case where the transmission power of the transmitter required by the wireless communication system is 18 dBm (63 mW) and distortion specifications to be satisfied by the transmitter is −30 dBc or less will now be described specifically.
At the time of MIMO transmission, both the first power amplifier and the second power amplifier operate. Each of the two power amplifiers outputs a signal with power that is half of the power to be transmitted by the transmitter. Therefore, each of the two power amplifiers outputs a signal with transmission power of 15 dBm (32 mW) which is half of 18 dBm. In addition, the first power amplifier and the second power amplifier need to be −30 dBc or less in distortion level at this time.
On the other hand, if only one transmitting series including, for example, the first modulation unit, the first power amplifier and the first antenna is used, then the first power amplifier is required to be capable of outputting a signal with power of 18 dBm at the time of non-MIMO transmission. Furthermore, the distortion level of the first power amplifier must be −30 dBc or less.
Typically, in current small-sized power amplifiers for wireless communication terminal, a class AB amplifier or a class B amplifier is usually used. The amplifier used in the ensuing description may be either of a class AB amplifier and a class B amplifier. Therefore, a class B amplifier is taken as an example in the ensuing description. According to characteristics of the class B amplifier, the operation efficiency theoretically becomes the highest at the saturation output level, and the operation efficiency falls as the output level becomes lower. On the other hand, the distortion level becomes higher as the output level approaches the saturation output level.
As described earlier, it is necessary to bring the power amplifier in the backoff region in order to suppress the distortion level below the specification value. However, the required backoff level also depends upon the modulation method of the modulated signal transmitted by the transmitter. In the ensuing description, it is supposed that the concrete backoff level for satisfying the specification that the distortion level should be −30 dBc is 10 dB in order to make the description easy to understand and simple.
The power level at the time of transmitting in non-MIMO is 18 dBm, and the backoff level needed to obtain distortion characteristics satisfying specifications is 10 dB. Therefore, the saturation output level required for the first power amplifier becomes 28 dBm. FIG. 16 shows theoretical characteristics of the efficiency of the class B amplifier as a function of the output power. In this class B amplifier, the saturation output level is 28 dBm and the operation efficiency at the time of saturation output is 45%. The abscissa indicates the output power level, and the ordinate indicates the operation efficiency. In this case, the operation efficiency becomes 14% when the first power amplifier is operating with the output level of 18 dBm.
On the other hand, at the time of MIMO transmission, the output level of the first power amplifier becomes 15 dBm. Therefore, the operation efficiency becomes 10% on the basis of the characteristics shown in FIG. 16. As compared with the case where the output level is 18 dBm, the operation efficiency is degraded by 4%. If the same amplifier as the first power amplifier is used as the second power amplifier as well, the operation efficiency of the second power amplifier becomes 10% in the same way.
If the transmitter for MIMO transmission is required to operate in non-MIMO, the transmitter needs to have a power amplifier having a high saturation output level as heretofore described. Since the backoff level of the power amplifier at the time of MIMO transmission increases, a problem that the operation efficiency is degraded and the power dissipation of the transmitter increases occurs.
If it is made possible to transmit a signal by using only one transmitting series in the transmitter for MIMO transmission, it becomes necessary to provide power amplifiers having a high saturation output level, resulting in a problem of increased power dissipation of the transmitter at the time of MIMO transmission.